I had to fight like eight of them at the same time on the 4th floor the Obel ruins. I just barely made it out of there. And also, I wasn't able to get the Ancient Parchment I was unlucky to get them since enemies on that floor apparently varies.

I believe I encountered them on the third floor, and they only appeared after I dug up the artifact that I was looking for. I still had all of my party members, so I ganged up on them with Cooperate attacks.